MILWAUKEE, Wis. — Wisconsin Gov. Scott Walker raked in at least $6 million in the last five weeks as he prepared to fight for his political life in the June 5 recall election, according to campaign finance reports released Tuesday.

The GOP governor’s latest haul puts his total fundraising at over $30 million since January 2011, with about $19 million coming in just this year.

Walker’s cash intake has dwarfed that of his opponent, Milwaukee Mayor Tom Barrett, who didn’t enter the race until late March and had to fend off a primary challenge. Barrett’s campaign reported raising $3.4 million in the past five weeks, bringing his total to $4.2 million since he began his campaign.

Both candidates’ fundraising totals could be substantially higher than indicated in the reports. Since May 21, the campaigns have only been required to disclose big last-minute donations larger than $500. Outside groups are also pouring cash into the election, which has taken on national significance.

The candidates had roughly the same amount of cash in their coffers heading into the final stretch. Walker reported $1.6 million in the bank on May 21; Barrett’s campaign had $1.5 million in the bank.

The governor’s formidable fundraising was thanks in large part to a loophole in the state law that allows the targets of a recall election to raise unlimited cash for expenses tied to the recall. He benefited from deep-pocketed GOP mega-donors from out of state looking to support his effort to limit unions’ collective bargaining rights.

The Walker campaign reported spending $20.8 million as of May 21.

Barrett hammered his opponent at several campaign stops on Tuesday for raking in cash from “out-of-state billionaires,” referring specifically to GOP donor Bob Perry of Texas, who gave $500,000 to Walker earlier this year.

“He doesn’t care one whit about Wisconsin Rapids, or Watertown, or Wauwatosa,” Barrett told supporters in Wisconsin Rapids. “What they care about is having this right-wing agenda that they can push through every single state.”

Walker’s campaign touted its grassroots support in a statement Tuesday, noting that most individual donors gave less than $50 during the last reporting period.

Still, Walker’s campaign continued to rake in big checks from wealthy supporters.

Richard Pieper, an executive at Pieper Electric in Milwaukee, donated $100,000 to Walker on Sunday. Other big donations included $50,000 from Richard Roberts, president of URL Pharma in Philadelphia, and $25,000 from Max Carney, CEO of Midwest Insurance in Springfield, Ill.

Barrett’s last-minute donations included a check last week for $43,128.00 from the International Association of Fire Fighters Wisconsin PAC, and $10,000 checks from Chicago attorney Keith Hebeisen, Geoffrey Seaman of Portland, Ore., and Ruth Kohler, director of an arts foundation in Sheboygan, Wis.